Image displaying apparatuses employing reflective spatial light modulators are capable of displaying images at high resolution and high contrast, and therefore, various types thereof have been developed and marketed.
The image displaying apparatuses are mostly three-plate color projectors employing three reflective spatial light modulators. The three-plate color projector separates a white beam emitted from a strong light source such as a metal halide lamp into three primary-color beams, guides the beams to the reflective spatial light modulators such as liquid crystal panels, respectively, drives the modulators with image signals of respective colors to modulate the beams, combines the modulated beams, and projects the combined beams to display an image.
FIG. 1 is a perspective view showing an optical system of an image displaying apparatus according to a related art.
The optical system shown in FIG. 1 is disclosed in, for example, Japanese Patent Application Laid-Open Publication No. H10-197949. This optical system has a two-layer structure. The upper layer of the optical system includes a light source 101 that emits a white beam. The white beam is passed through a collimator lens 102 and becomes substantially a parallel white beam. The parallel white beam is passed through an integrator 103, a cold mirror 104, and an infrared cut filter 105 and is made incident to a three-color-separation cross dichroic prism 106. The prism 106 separates the parallel white beam into three primary-color beams, i.e., red beam (R-beam), green beam (G-beam), and blue beam (B-beam) and emits these beams in three directions, respectively. The emitted R-, G-, and B-beams are made incident to polarizing beam splitters 107r, 107g, and 107b, respectively. Each of the splitters 107r, 107g, and 107b has a polarizing reflective face that reflects only an s-polarized component of the incident beam. The reflected beam components from the polarizing reflective faces of the splitters 107r, 107g, and 107b are emitted as parallel beams in a downward direction.
FIG. 2 is a perspective view showing an essential part of the optical system of FIG. 1.
In FIG. 2, the beams emitted from the polarizing beam splitters 107r, 107g, and 107b are passed through convex lenses 108r, 108g, and 108b and polarizers 109r, 109g, and 109b and are made incident to polarizing beam splitters 110r, 110g, and 110b of the lower layer, respectively. The incident beams are reflected by polarizing reflective faces of the splitters 110r, 110g, and 110b and are made incident to reflective spatial light modulators 111r, 111g, and 111b, respectively.
The reflective spatial light modulators 111r, 111g, and 111b modulate the incident beams according to image signals of respective colors supplied to the modulators 111r, 111g, and 111b and polarize and reflect the modulated beams. In FIG. 1, the reflected beams polarized and modulated by the modulators 111r, 111g, and 111b are transmitted through the polarizing beam splitters 110r, 110g, and 110b, respectively, and are made incident to a three-color-combining cross dichroic prism 112.
The three-color-combining cross dichroic prism 112 combines the incident beams of the respective colors and emits the combined beams to a projection lens 113. The projection lens 113 projects the beams onto a screen (not shown) to display an image.
This image displaying apparatus has a problem of lowering the contrast of a displayed image when incident beams have large angles with respect to optical axes of the polarizing beam splitters 110r, 110g, and 110b. 
To solve the problem, wave plates 114 are arranged between the reflective spatial light modulators 111r, 111g, and 111b and the polarizing beam splitters 110r, 110g, and 110b, respectively. The wave plates 114 are each a quarter wave plate. Each of the wave plates 114 is arranged so that a fast axis or a slow axis thereof is orthogonal to a plane that includes an incident optical axis and a reflective optical axis of the polarizing beam splitter. The wave plates 114 improve the contrast of a displayed image.
The details of contrast improvement are described in Japanese Patent Application Laid-Open Publication No. 2000-206463. Namely, a beam made incident to a polarizing beam splitter may have an angle with respect to a plane (incident plane) containing an incident optical axis and a reflective optical axis of the splitter. When such an oblique beam is reflected by a reflective spatial light modulator and is again made incident to the splitter, the wave plate corrects a polarization direction of the beam. This correction makes the beam to be completely reflected by the splitter when the beam is to display a black color, to thereby improve the contrast of a displayed image.
In this way, the quarter wave plates prevent the lowering of the contrast of a displayed image when beams are obliquely made incident to the polarizing beam splitters. With the quarter wave plates, the image displaying apparatus can increase the spreading angles of beams made incident to the splitters, to display bright and high-contrast images.
The quarter wave plate is made by attaching a polymeric film made of polyvinyl alcohol or polycarbonate to one surface of a glass substrate through an adhesive layer or a bonding layer. Alternatively, such a polymeric film may be sandwiched between two glass substrates.
The polymeric film is produced by stretching polymeric material in an axial direction. By adjusting a stretching factor, it is possible to adjust the refractive indexes of the film in the stretching direction and in a direction orthogonal to the stretching direction, as well as controlling the thickness and phase difference of the film.
Polymeric materials generally show a large refractive index in a stretching direction and have a positive intrinsic birefringence (Δn>0). A direction in which a refractive index increases is referred to as a slow axis, or simply as an optical axis. A direction orthogonal to the slow axis is referred to as a fast axis.
Due to the uniaxial stretching, the polymeric film has a phase characteristic and easily changes a phase difference in response to mechanical stress. If a temperature change occurs, it may cause mechanical stress through thermal expansion, to change the phase difference characteristic of the polymeric film.
For example, an increase in the intensity of the light source in the image displaying apparatus leads to increase the temperature of optical parts. Heat absorption of the optical parts causes mechanical stress that may cause reversible change or irreversible deterioration on the phase difference characteristics of the wave plates.
In addition, the wave plate with the polymeric film has a problem in connection with in-plane uniformity due to the phase difference.
There is a wave plate that employs crystals such as quartz crystals. Quartz wave plates are disclosed in, for example, Japanese Patent Application Laid-Open Publications No. 2003-222724 and No. 2003-302523 and in a catalogue (Laser & Optics Guide: Optical Parts) of MELLES GRIOT. The quartz wave plates (quarter wavelength) have no problems mentioned above and can be employed to form an image displaying apparatus capable of withstanding a high-output light source to display bright images.
The quartz wave plates are classified into first-order quarter wave plates and multiple-order quarter wave plates. The first-order quarter wave plates provide a beam having a phase difference of just a quarter wavelength. The multiple-order quarter wave plates provide a beam having a phase difference of a quarter wavelength plus an integer multiple of a wavelength. It is preferable for wave plates used for image displaying apparatuses to produce a phase difference of quarter wavelength for a separated color beam. In this regard, the first-order quarter wave plates are preferable for image displaying apparatuses.
To form a first-order quartz wave plate from a single quartz plate, the quartz plate must have a thickness of 10 μm to 20 μm. For the present polishing technology, it is difficult to practically produce such plates. Therefore, the first-order quarter wave plate is made by laminating two elemental quartz plates with slow axes thereof being oriented substantially orthogonal to each other.
In such a laminated quartz wave plate, the slow axes of the elemental quartz plates must be orthogonal to each other. Perfectly orthogonalizing the slow axes of elemental quartz plates, however, is difficult to achieve economically.